1. Field of the Invention
The present invention relates to an acoustic flow meter and in particular to an ultrasound flow meter useable in the monitoring of gas flow in breathing aid devices such as ventilators or respirators.
2. Description of the Prior Art
Flow meters in which the time of flight of an acoustic (usually ultrasonic) pulse is used to determine the velocity (and hence the flow rate) of the fluid through which the pulse was transmitted are well known in the art. Devices such as those described in PCT Application WO 94/28790 and U.S. Pat. No. 5,247,826, improve on this basic methodology by arranging for the transit times of ultrasonic pulses to be measured both upstream (T.sub.u) and downstream (T.sub.d) of the fluid flow. These transit times are then supplied to a microprocessor which is set to calculate the fluid flow rate using standard algorithms. A thermometer is also included in both devices to measure the ambient temperature of the fluid. Since the velocity of sound in a medium changes with its temperature a more accurate transit time can be derived with a knowledge of the ambient temperature.
In PCT Application WO 94/28790 a pair of cells, each having a piezoelectric transmitter and receiver, are placed so that an ultrasonic pulse can travel between the cells at an angle to the direction of fluid flow. By having the transmitter in each cell transmit an ultrasonic pulse for reception by the receiver the other cell, both T.sub.u and T.sub.d can be measured. The device described in U.S. Pat. No. 5,247,826 achieves the same result by arranging for a pair of ultrasonic transceivers, which are spaced apart in an elongate coiled tube through which gas can flow, to alternately operate as transmitters and receivers.
A piezoelectric crystal does not emit a single pulse when energized with a single electrical pulse. Rather the crystal is caused to oscillate at a characteristic resonant frequency to emit a "packet" that comprises a number of pulses. The envelope of the transmitter signals decays rapidly with time, usually producing a train of six or so cycles. Thus small errors in the determination of the flow rate may result if the determination is made using different pulses from within the packet.
A problem may therefore arise when conventional devices are used in situations where it is critical to maintain flow rates within fine tolerances, for example in medical applications such as monitoring breathing gas flow rates in ventilators and respirators. In these applications flow meters must be capable of accurately and reliably detecting small changes in gas flow rates. Conventional devices, however, may record small changes which on the face of it look correct but which do not actually result from flow rate changes but rather from registering the arrival time of the wrong acoustic pulse from within a particular packet.